Until we live in a eutopia, there will be aggressors, and there will be the weak. There will be those seeking power/control, motivated by greed, sometimes just motivated by survival instincts. All men are NOT created equal. There will always be those who are motivated to improve their life, some by any means necessary. Those who do not have will want, and some will try to get what they want- by ANY MEANS NECESSARY! Those who have, will try to keep. They will try to keep by whatever means are necessary. Is dying for your country (read home, if you will) a good idea? No, of course not. Is protecting your way of life a good idea? Yes. I don't want to be forced to bow to any gods (political, national, religious, ideological, etc.)and I will fight those who will try to force me, even if I have to fight my own country (read: Nation). The issue is not black and white by any means. I will protect my home, hopefully not having to die while trying to do so and at the same time hoping that I'll have the stones to sacrifice my life if it means that I will be prolonging the life of those I love, and those who are incapable of doing so on their own. Would I kill someone (if possible) trying to just steal my car? No. Would I kill someone trying to steal my car while my child was still in it? Yes. Would I kill someone who was threatening my own life? Yes, if possible. If I had the time to convince them not to kill me without me killing them would I take that route? If it was feasible.
While I am sure that there are indeed some true pacifists in the world, I don't tend to believe that most who claim to be pacifist are really so. Many people claiming to be pacifist are pacifist at the expense of others and I am as disgusted with them as I am the "warriors". The question in the subject line is a bullshit question. The question is loaded. Many animals travel in packs. Most of these animals have a hierarchy. They are equivalent to human tribes. They all have there own jobs. The weak are eventually weeded out. They either learn to survive, or they are abandoned by the herd, eventually.
Edited to add- I am also an atheist.